Conventionally, projectiles are accelerated within the barrel by the action of a so-called chemical propellant, i.e. by propulsion gases generated by the rapid combustion of chemical propellants. However the muzzle velocity of these projectiles will usually be below 1500 m/sec. which is due to the low sound speed in the combustion gases. With some type of known chemical propellant ammunition it is possible to reach muzzle velocities of up to 2000 m/sec. but this requires relatively large quantities of propellant per projectile and gives rise to considerable stresses in the breech and barrel.
With the ignition of a chemical propellant the combustion process is started and the gaseous products are produced at a rate of w defined by EQU w=AbP.sup..alpha. (I)
where A is the surface area of the propellant, P is the pressure within the gun chamber and b and .alpha. are burning coefficients specific to each type of propellant. When the force on the projectile base becomes larger than the static friction and the engraving forces, the projectile starts moving and the volume in which the gas expands increases. This increase of volume results in a general decrease of the average pressure P.sub.av in the barrel after a certain maximum is reached. The pressure P.sub.b behind the advancing projectile is further reduced to values lower than P.sub.av due to the gas rarefaction behind the "escaping projectile". The pressure P.sub.b acting on the moving projectile base is determined approximately by the equation ##EQU1## where .gamma. is the ratio cp/cv of the propellant gases specific heats at constant pressure and constant volume, respectively, M is the Mach number of the projectile with relation to the propelling gas and .phi. is an experimental constant usually in the range of 1.5. It follows from equation (II) that the higher the projectile velocity the lower the P.sub.av to P.sub.b ratio and the pressure exerted on the projectile base decreases faster for high velocity guns than for low velocity guns. Consequently, the rate of acceleration in high velocity guns decreases faster than in low velocity guns and so does their thermal efficiency.
In order to overcome the intrinsic limitation of chemical propellants, several guns have been proposed which are characterized by supplementing the initial propulsion from the propellant ignited at the breech by a plurality of successively acting booster propulsions brought about by additional chemical propellant charges mounted along the barrel and adapted to be initiated by the passing projectile. Arrangements of this type are described, for example in U.S. Pat. Nos. 2,360,217; 3,044,363; 3,357,306; 3,459,101 and 3,613,499.
By using this technique it is possible to obtain higher velocities due to the additive actions of the initial and booster charges. Nevertheless, these guns have not enjoyed widespread use, mainly owing to the difficulty of accomplishing the required control of the burning of the booster charges along the barrel.
Another technique to accelerate projectiles to hypervelocity is the use of highly pressurized light gases, "the light gas cannon". The light gas cannon too did not develop beyond the experimental stage.
Various proposals are known for the acceleration of projectiles by means of electrothermal energy. Thus, for example U.S. Pat. Nos. 2,783,684 and 2,790,354 describe methods and means for accelerating a projectile within a gun's barrel by generating high pressure waves which accelerate the projectile down the length of the tube. The high pressure is maintained by means of electric arcs generated within the tube via high voltage electrodes spaced along the length of the tube, so that the electric arcs will continuously be generated as the projectile travels down the tube.
By way of a further development of the concept of accelerating projectiles by means of electrothermal energy, the use of plasma has repeatedly been proposed such as in U.S. Pat. Nos. 3,916,761, 4,590,842, EP-A2-0232594 and U.S. Pat. No. 4,715,261. In accordance with all these proposals chemical propellants are replaced by a plasma, or a gas heated by plasma acting on the rear of the projectile. The major limitation for practical application of the known plasma propellant is the very large and cumbersome electrical power supply that is associated therewith.
There are also known so-called magnetic rail gun accelerators with plasma propellants as disclosed for example in U.S. Pat. Nos. 4,343,223; 4,467,696; 4,485,720; 4,577,545 and 4,621,577.
In our U.S. Pat. No. 5,016,518, issued May 21, 1991, we are describing for the first time a gun for accelerating projectiles in which the travelling chemical charges are ignited by electrothermal energy sources. By one mode disclosed in that patent application there are provided electrothermal energy injectors along the gun barrel which are fired synchronously with the displacement of the projectile within the barrel, each such injector igniting a distinct chemical propellant charge attached to the projectile. Essentially that mode of the gun of our U.S. Pat. No. 5,016,518, issued May 21, 1991, operates by the travelling charge principle in which the boosting of the thrust on the projectile is brought about by successively ignited propellant charges attached to the projectile itself while the electrothermal energy injectors on the barrel serve for ignition only.
Our U.S. Pat. No. 5,016,518, issued May 21, 1991, further describes an alternative method by which a plasma injector unit is mounted at the rear of the gun coaxially with the barrel and the injected plasma acts via a working fluid to initiates the chemical propellant.
It is the object of the present invention to provide an improved gun in which high muzzle velocities can be reached and which is expected to perform better than the various known guns based on chemical and/or electrothermal acceleration.